Abstract

Ants and plants are engaged in thousands of mutualisms, which can be categorised in two major groups: dispersal and defence
mutualisms. Myrmecochory, the dispersal of plant seeds by ants, is usually facilitated by the elaiosome, an appendix of the
seed that is used as a reward. Because elaiosomes represent a nutritional supplement rather than a full diet, dispersal mutualisms
are facultative ones. Only in ant‐gardens, the ants use the dispersed epiphytes as support to construct their carton nests,
and these mutualisms tend to be obligate ones. In facultative defensive mutualisms, plants provide ant rewards, such as cellular
food bodies (FBs) or extrafloral nectar (EFN), to foraging ants that serve as an indirect defence against herbivores. Obligate
defensive mutualisms, by contrast, are based on the provisioning of nesting space. Ants dominate most terrestrial ecosystems.
Owing to their mobility and social structure, they appear perfect partners for plant dispersal and defence.

Key Concepts:

Plants can benefit from indirect defence that is mediated through the third trophic level.

Ants are the dominant animal group in terrestrial ecosystem and – being common predators – have a high potential to defend
plants against herbivores.

Plants can provide food resources or nesting space to ants in order to increase their presence and capacity to function as
defenders.

Obligate defensive ant–plant mutualisms. In the majority of obligate ant–plant mutualisms, specialised defensive ant species
are housed and nourished by specialised myrmecophytes, which depend on the ant‐mediated defence for their survival. Ant‐exclusion
experiments revealed similarly dramatic effects on leaf damage for Acacia hindsii in Mexico (a) with ants; (b) without ants; and for Macaranga bancana in Malaysia ((c), left plant with ants, right plant after six weeks of ant exclusion). Ants are nourished commonly by cellular
food bodies (FBs) ((d), Pseudomyrmex gracilis worker harvesting an FB of Acacia cornigera) and extrafloral nectar (EFN) ((e), Pseudomyrmex peperi ants feeding on EFN of Acacia hindsii). FBs can be produced on different plant parts ((f), FBs on the pouches below leaf stalks of Cecropia mexicana; (g, i), FBs under recurved stipules of M. bancana and (h), FBs on upper surfaces of stipules of Macaranga hosei). In several systems where EFN is missing, scale insects are kept as a third partner ((g), Crematogaster sp. workers attending scale insects within the hollow twig of M. bancana, which serves as domatium in this species).
Reproduced in part from Heil M , with permission from Wiley‐Blackwell.

Heil M,
González‐Teuber M,
Clement LW et al.
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Divergent investment strategies of Acacia myrmecophytes and the coexistence of mutualists and exploiters.
Proceedings of the National Academy of Sciences of the USA
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Heil M,
Koch T,
Hilpert A et al.
(2001)
Extrafloral nectar production of the ant‐associated plant, Macaranga tanarius, is an induced, indirect, defensive response elicited by jasmonic acid.
Proceedings of the National Academy of Sciences of the USA
98:
1083–1088.